Tin Whiskers are a crystalline growth made of the element tin.
Tin whiskers often form on lead free solder used to fix components to circuit boards since lead was banned.
They are in the form of a very thin straight hair with one end attached to the tin from which they are formed. In my case they have grown on a piece of tin plated steel used to screen electronic components which are part of a central heating wireless thermostat radio receiver. This receiver has developed an intermittent problem and doesn’t switch on my central heating when it should. I suspect there is an electrical fault which may be caused by a tin whisker growing inside the component screening box until it touched some electrical connection.
Since I have not completely dismantled the item to find out for sure I don’t know if that is correct but as you can see from the pictures the Tin Whiskers that I have found on the outside of the screening box (see featured image) are up to 2mm long. Although I already knew about Tin Whiskers these are the first I have seen in 50 years of handling electrical or electronic devices. I have used a USB connected microscope to photograph them but I have had a lot of difficulty. They are so thin they are hardly visible with a watchmakers eyeglass with 10x magnification.
The tin plated steel shown here is covered in minor scratches which I think have opened the surface to the formation of Tin Whiskers. There are many small but visible stumps along these scratches which seem to be the beginning of whiskers. See below: